Edit attachment fields

function pmc_variety_image_credit( $form_fields, $attachment ) {

	$image_credit = array(
		'label' => 'Image Credit',
		'value' => get_post_meta( $attachment->ID, '_image_credit', true ),
		'helps' => 'Image credit, e.g. "Copyright ' . date('Y') . ' Getty Images".',
		'show_in_edit' => true,
	);

	$form_fields['image_credit'] = $image_credit;
	
	return $form_fields;
}
add_filter( 'attachment_fields_to_edit', 'pmc_variety_image_credit', 10, 2 );

function pmc_variety_image_credit_save( $post, $attachment ) {
	if ( isset($attachment['image_credit']) ) {
		$image_credit = trim( strip_tags( $attachment['image_credit'] ) );
		update_post_meta( $post['ID'], '_image_credit', $image_credit );
	}

	return $post;
}
add_filter( 'attachment_fields_to_save', 'pmc_variety_image_credit_save', 10, 2 );